Theoretical analysis of bearing mechanism and engineering application of pipe roof in a highway tunnel.

Establishing a reasonable and precise theoretical model for pipe roof is crucial for advancing pre-support technology in tunnel construction. By considering the processes of tunnel excavation and support, as well as the reduced constraint reaction force on the pipe roof in unexcavated section due to the disturbance at the tunnel face, this paper establishes a load-structure model for pipe roof based on the Euler-Bernoulli beam theory. The Pasternak elastic foundation model is utilized to ascertain the constraint reaction force exerted by primary support and surrounding soil ahead of the tunnel face on the pipe roof. Analytical expressions describing the load-bearing behaviour of the pipe roof under during each cycle of excavation and support are derived. The load-bearing behaviour of the pipe roof as the tunnel face advances to any position is solved via the superposition method. The accuracy and validity of the model are confirmed through case calculations and comparisons with existing research results. Furthermore, the effects of circumferential spacing, steel pipe diameter, primary support strength, and surrounding soil strength on the load-bearing behaviour of the pipe roof are examined. It is suggested that improving the surrounding soil strength can effectively enhance the effectiveness of pipe roof pre-reinforcement. The findings of this study offer valuable insights for evaluating the rationality of pipe roof design and support its design and construction processes.